The present invention relates to an organic light-emitting device used for display devices, specifically, to an organic light-emitting device which can be driven by a low drive voltage with low power consumption. The present invention relates also to a method of manufacturing the organic light-emitting device.
FIG. 17(a) is a cross sectional view of a conventional organic light-emitting device including a binary organic film laminate, and FIG. 17(b) is a cross sectional view of another conventional organic light-emitting device including a ternary organic film laminate.
Referring now to these figures, the organic light-emitting device includes a transparent substrate 1, an anode 2 on the transparent substrate 1, an organic film laminate 3 on the anode 2, and a cathode 4 on the organic film laminate 3. The anode 2 is made of a transparent and conductive film. The cathode 4 is made of a metal film. In FIG. 17(a), the organic film laminate 3 consists of a hole transport layer 3h and a light-emitting layer 3r. In FIG. 17(b), the organic film laminate 3 consists of a hole transport layer 3h, a light-emitting layer 3r and an electron transport layer 3e. The hole transport layer 3h facilitates hole injection from the anode 2 to the light-emitting layer 3r and blocks electron injection. The electron transport layer 3e facilitates electron injection from the cathode 4 to the light-emitting layer 3r. A driving power supply E is connected to the anode 2 and the cathode 4. Arrows in FIGS. 17(a) and 17(b) indicate the directions of light emissions from the respective light-emitting devices. Recently, still another organic light-emitting device including a quaternary organic film laminate having a hole injection layer between the anode and the hole transport layer has been proposed.
Research and development of the organic light-emitting device have been conducted vigorously, since the organic light-emitting device exhibits excellent visibility due to the self-light-emitting nature thereof and the organic light-emitting device can be driven by a low voltage.
The mechanism for light-emission from the organic light-emitting device has been considered as follows. An electron injected from the cathode and a hole injected from the anode recombine in the vicinity of the boundary between the hole transport layer and the light-emitting layer, and an exciton is generated. Light is radiated during the radiative deactivation of the exciton. The light is radiated outside through the anode, that is a transparent and electrically conductive film, and the transparent substrate.
The organic light-emitting device of a simple matrix type (hereinafter sometimes referred to as "passive matrix type") has the simplest structure which includes many picture elements. FIG. 18(a) is a top plan view of a conventional organic light-emitting device of simple matrix type. FIG. 18(b) is a sectional view taken along line 18(b)--18(b) in FIG. 18(a). FIG. 18(c) is a partly-broken perspective view of the conventional organic light-emitting device of FIG. 18(a).
The passive-matrix-type device includes anodes 2 consisting of strips extending parallel to each other on a transparent substrate 1, cathodes 4 consisting of strips extending perpendicularly to the anode strips, and an organic film layer 3 between the electrodes 2 and 4. An area where a strip of the anode 2 and a strip of the cathode 4 cross each other is one unit of a light-emitting area, that is one picture element. A plurality of the picture elements is arranged in a display area D. The peripheral area of the substrate where the anode strips and the cathode strips are extended outwardly from the display area D is a connection area C. A display device is constructed by connecting the display area D to an external drive circuit via the connection area C. Therefore, the anodes 2 and the cathodes 4 work also as the wirings.
In these days, it has been required for the display devices to have a wider display screen and a much finer structure. To meet these recent requirements, the wirings in the display area of the organic light-emitting device are becoming longer and finer. Usually, a metal oxide, such as indium tin oxide (hereinafter referred to as "ITO") and indium zinc oxide (hereinafter referred to as "IZO"), is used for the anode. Also, a metallic material, such as Al and aluminum alloys, is used for the cathode. The resistivity of the metal oxide is higher than that of aluminum and so on. Although it is possible to lower the wiring resistance of the transparent conductive film by increasing the thickness thereof, it is not desirable to reduce the transparency of the transparent conductive film by increasing the thickness thereof. Therefore, the wiring resistance of the transparent conductive film for the anode tends to be higher. A higher drive voltage necessary for compensating the voltage drop across the high wiring resistance of the anode in driving the display panel causes more power consumption. Since the joule heat generated from the wirings heats the organic layer, the properties of the organic light-emitting device are deteriorated and the life of the organic light-emitting device is shortened.
Japanese Unexamined Laid Open Patent Publications No. H05-307997 and No. H06-5369 disclose a lamination of a transparent conductive film and a metal film for reducing the resistance of the anode.
Japanese Unexamined Laid Open Patent Publication No. H05-307997 describes that the metal film interposed partially between the anode and the hole transport layer has a work function smaller than that of the anode. Japanese Unexamined Laid Open Patent Publication No. H06-5369 describes that the anode consists of a transparent first anode portion and a second anode portion contacting with the hole transport layer and having a work function larger than that of the first anode portion. Thus, the wiring resistance is reduced by laminating the metal film to the anode, and the power consumption efficiency of the display device is improved by utilizing the work function of the metal film, so that controlled carrier injection from the anode to the organic film laminate may be facilitated.
Japanese Unexamined Laid Open Patent Publication No. H06-5369 discloses a lamination of a very thin film of a material exhibiting a high hole injection efficiency so that the transparency thereof is 90% or more. Ni, Se, Pd, Ir, Pt and Au are described as examples of the materials which exhibit a high hole injection efficiency. In manufacturing the device, an Au film of 2 nm in thickness or a Pt film of 2 nm in thickness is deposited on the entire upper surface of an ITO film of 100 nm in thickness. As a result, the drive voltage is lowered, and the light-emission efficiency of the device is improved from 3.0 lm/W to 3.2 lm/W and 3.6 lm/W, respectively.
Although no clear descriptions have been made on the wiring resistance, the very thin metal film of around 2 nm in thickness usually has an island structure. Since the very thin metal film is not uniform due to the island structure, the resistivity of the very thin metal film tends to be higher than that of the metal film which is thick enough. Therefore, the wiring resistance is not reduced sufficiently when the display area is wide or when the display area includes many picture elements.
Although it is necessary to thicken the metal film for reducing the wiring resistance, the thickening of the metal film is not always effective, since the transparency for the visible light is reduced and the light-emitting efficiency of the device is lowered.
According to Japanese Unexamined Laid Open Patent Publication No. H05-307997, the anode is constructed by laminating a metal film, which has a work function smaller than that of the transparent conductive film, on a part of the transparent conductive film.
The metal film having a small work function partially blocks the hole injection and reduces a current pertinent to the emission of light which is not radiated outside. When the work function of the metal is the same as or larger than that of the transparent conductive film, light is emitted by the hole injection to the organic layer due to the hole injection capability of the metal. Since the metal film works as a shield film, the emitted light is not radiated outside and power consumption increases due to the light emission loss. However, since electrons are injected easily from the metal film with a small work function, cross talk may be caused by the light emitted even when a reverse bias voltage is applied to stop the light emission. When a metal film is disposed partly underneath the transparent conductive film, current loss is also caused, since the metal film works as a shield film.
In the examples as disclosed in Japanese Publication No. H05-307997, combinations of materials having a large work function, such as ITO film (work function is about 5.0 eV), with metals, which have the work functions of 4.3 eV or smaller, such as Al, Mg, In and Ag, their alloys and appropriate combinations of these metals and alloys, are described. It is said that using metal with a smaller work function is preferable.
However, it has been known that when Al for the metal and ITO for the transparent conductive film are used, the ITO film and the Al film are corroded in the photolithographic process, that is the development process of the photoresist which uses an Al mask for etching, and the reliability of the electrodes is greatly lowered (cf. H. Nishino et al., Sharp Technical Report, Vol. 44 (1990), pp. 31-36).
The same phenomena are observed when an aluminum alloy is used. It is considered that the foregoing phenomena are caused by the formation of a local cell, wherein materials with the large difference in the work function work as positive and negative electrodes and a developing agent, e.g. TMAH ((CH.sub.3).sub.4 NO solution), works as an electrolyte solution.
Therefore, the foregoing phenomena may be caused not only in the development process of the photoresist but also in the wet etching process of the electrode materials when the materials with the large difference in the work function are laminated.
Therefore, it is highly possible that the material combinations exemplary described in Japanese Unexamined Laid Open Patent Publication No. H05-307997 cause corrosion of the transparent conductive film and the metal film. Corrosion of the transparent conductive film and the metal film should be taken into account also when a material having a work function larger than that of the transparent conductive film is combined with the transparent conductive film.
In view of the foregoing, it is an object of the invention to provide an organic light-emitting device which obviates the foregoing problems.
It is another object of the invention to provide an organic light-emitting device, wherein a wiring resistance of an anode is small, a light-emitting efficiency thereof is high, and a power consumption thereof is small.
It is a further object of the invention to provide an organic light-emitting device which does not cause illuminance deviation by a voltage drop, does not emit light under a reverse bias voltage, and, therefore, prevents cross talks.
It is a still further object of the invention to provide a method of manufacturing such the organic light-emitting devices with high throughput.